I second andyroo's advice
But, assuming you wish to remain with a PC, how much RAM do you have and which version of Windows?
A number of my friends have found that often you just need to wipe the system and start over from the install discs when cruft like that creeps in ... all those .dlls from bad installs, s00p3r s3kr1t spyware, corrupted system files, etc., just build up. When my Dad's Win98 PC started going bananas to the point where it blue-screened five minutes after starting, we just wiped the whole system and started over with *just* the applications we were using. His system has been relatively stable since then since we only installed the programs we were actually using on it.
You may be able to stave that off with adding some more RAM, or getting an upgrade CD if you're running an older version of Windows and upgrade Windows to that version, which is like reinstalling Windows with less inconvenience of having to backup your files, destroy the hard disk, and copy them back after the re-install. (You may have to do this still but there should be less trouble with lost apps and data files.)
Also, if your PC is aging, perhaps some components are failing, too. Cruft invades hardware and software alike.
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